The present invention relates to oil cooled x-ray tubes and to means for extending the service life of x-ray tubes used in x-ray generating equipment for diagnostic and therapeutic radiology such as computerized axial tomography.
X-ray tubes are normally enclosed in an oil-filled protective casing. A glass or metal envelope contains a cathode plate, a rotating disk target and a rotor that is part of a motor assembly that spins the target. A stator is provided outside the tube proximate to the rotor and overlapping therewith. The glass envelope is enclosed in an oil-filled lead casing having a window through which the x-rays that are generated escape the tube. The casing in some x-ray tubes may include an expansion vessel, such as a bellows.
X-rays are produced when, in a vacuum, electrons are released, accelerated and then abruptly stopped. This takes place in the x-ray tube. To release electrons, the filament in the tube is heated to incandescence by passing an electric current through it. The electrons are accelerated by a high voltage, ranging from about ten thousand volts to in excess of hundreds of thousands of volt, between the anode and the cathode and impinge on the anode, whereby they are abruptly slowed down. The anode, usually referred to as the target, is often of the rotating disc type, so that the electron beam is constantly striking a different point on the anode perimeter. The x-ray tube itself is made of glass or metal, but is enclosed in a protective casing that is filled with oil to absorb the heat produced. High voltages for operating the tube are supplied by a transformer. The alternating current is rectified by means of rectifier tubes or barrier-layered rectifiers.
For therapeutic purposes, e.g., the treatment of tumors, the x-rays employed are in some cases generated at much higher voltages (over 4,000,000 volts). Also, the rays emitted by radium and artificial radiotropics, as well as electrons, neutrons and other high speed particles produced by a betatron are used in radio therapy.
The present invention includes an x-ray tube comprising: a glass or metal envelope; a cathode operatively positioned in the glass envelope; an anode assembly including a rotor, a stator, operatively positioned relative to the rotor, and a target or anode operatively positioned relative to the cathode and connected to the rotor.
Another aspect of the present invention is an x-ray system comprising; an enclosure having dielectric cooling oil contained therein; an oil pump positioned relative to the enclosure for circulating the oil within the system; at least one cooling means connected to the enclosure and the oil pump, for cooling the oil; an x-ray tube, positioned inside the enclosure, for generating the x-rays, the x-ray tube comprising: a glass or metal envelope; a cathode positioned in the glass frame; an anode assembly including a rotor, a stator positioned relative to the rotor, and a target positioned relative to the cathode and connected to a stem.
Dielectric oils possess a dielectric breakdown strength of 80 kv for a 40 mil gap when the oils are clean and dry. However, the breakdown strength decreases rapidly when the oil becomes contaminated with moisture, particles, or polar chemicals. For example, the dielectric breakdown strength may be reduced by as much as 50% by addition of 35 ppm water, 50 ppm citric acid, or less than 0.7 mg carbon powder/liter oil.
Tube assembly requires the use of numerous machined and/or formed plastic and metal components, plus a pump, motor, and heat exchanger along with the x-ray tube insert. As the dielectric oil circulates through and around these components to cool the insert it can become contaminated with particles, mold release, machining fluids, moisture, etc. dissolved or extracted from these materials. These impurities will often lower the dielectric breakdown strength of the oil. Additionally, oxidation of the oil from exposure to radiation in the presence of traces of dissolved air and/or H2O will lead to the formation of carboxylic acids and high molecular weight oil xe2x80x9csludgesxe2x80x9d. These generated impurities will also decrease the oil dielectric strength with operating time. Occasionally the dielectric strength will decrease sufficiently to produce an arc from one of the high voltage leads to the casing resulting in further damage to the oil, the generation of gaseous products such as acetylene, and a reduction in tube life. For this reason, the dielectric strength of the cooling oil must be maintained as high as possible.
A representative x-ray system embodying the present invention in one preferred form thereof generally includes an oil pump, an anode end, a cathode end, a center section positioned between the anode end and the cathode end, which contains the x-ray tube. A radiator or heat exchanger for cooling the oil is positioned to one side of the center section and may have fans operatively connected to the radiator for providing cooling air flow over the radiator.
During operation, typically, x-ray tube assembly generates a planar beam of radiation which is then rotated around the body. Various detectors, located around the patient, detect the intensity of the beam. Detectors are connected to a computer which, based on intensity readings, generates an image of a slice of the body. The patient is then moved longitudinally through a gantry with the x-ray tube assembly generating image slices so that the computer can form a three-dimensional image of the body.
In the course of operation, much heat is generated by x-ray tube assembly 4 and this heat must be removed if the service life of the x-ray tube is not to be unduly reduced. As described above, it is known to cool x-ray tubes by circulating a fluid, typically oil, within the tube and externally through a cooling system to remove as much heat as possible. In addition to being used as vehicle for cooling, the fluid is also used for its dielectric properties in order to insulate the anode connection from ground and the cathode connection. A representative x-ray system embodying the present invention in one preferred form thereof generally includes an oil pump, an anode end, a cathode end, a center section positioned between the anode end and the cathode end, which contains the x-ray tube. A radiator or heat exchanger for cooling the oil is positioned to one side of the center section and may have fans operatively connected to the radiator for providing cooling air flow over the radiator.
Even employing this type of fluid for purposes of cooling and electrical insulation, x-ray tubes have a finite service life. There are several causes of x-ray tube failure, most of which are related to thermal characteristics of the x-ray tube. Hence, heat removal is an important concern in attempting to extend the service life of an x-ray tube.
A first type of tube failure is related to excessive anode temperature during a single exposure which may result in localized surface melting and pitting of the anode.
A second type of tube failure results from maintaining the anode at elevated temperatures for prolonged periods. If the thermal stress on an x-ray tube anode is maintained for prolonged periods, such as during fluoroscopy, the thermal capacity of the total anode system and of the x-ray tube housing is the limitation to operation.
Coolant fluid, due to continuous heat and repeated arcing, will eventually break down. When the oil breaks down its dielectric properties as well as its ability to carry away heat (i.e. viscosity) are adversely affected. This results in less electrical insulation between the anode connection and ground connections (and/or the cathode connection) which leads to more arcing and, eventually, tube failure. Hence, proper electrical insulation (i.e., maintaining the proper dielectric property of the coolant fluid) is also an important concern in attempting to extend the service life of an x-ray tube.
Oil is circulated within an x-ray tube casing to cool the tube target thereby extending the time that x-rays can be generated. Since x-ray tubes operate at a high voltage, between 75 to 150 kv, the oil must also be of high dielectric quality to prevent arcing from a high voltage lead to the casing. Incorporation in the cooling circuit of a filter into the oil circulation loop removes contaminants which reduce the dielectric breakdown strength of the oil and thereby extend the service life of the x-ray tube.
The purpose of the invention is to to provide a method for removing harmful impurities which does not require that the x-ray tube be removed from the equipment or the hospital. This is accomplished by incorporating and operating an adsorbing filter within the oil cooling and circulation loop.